The BALB/3T3/A31-1-1 mouse embryo cell line, previously established as a model for quantitative cytotoxicity and neoplastic transformation, was used to study the effects induced by samples of crystalline silica (<5 microm in diameter) alone or mixed with other mineral dusts. Test particulates, characterized for purity, surface properties and size, included several samples of silica (quartz, cristobalite and tridymite), hematite (ferric oxide), and also anatase and rutile (two titanium dioxide polymorphs). Silica uptake, cellular localization and damage to organ- elles were studied by scanning and transmission electron microscopy. Silica particles <5 microm were localized in the cytoplasm, mostly in membrane-bound vacuoles; a few particles, smaller than 0.4 microm, were localized inside nuclei. Further transformation assays of quartz (Min-U- Sil 5) alone confirmed the dose dependent induction of transformed foci up to a dose of 25 microg/cm2, followed by a plateau indicative of a rate- limiting factor. When tested alone, hematite and anatase were not cytotoxic, whereas rutile was more toxic than quartz; when tested in a 1:1 mixture with quartz, anatase markedly inhibited both the toxicity and the transforming activity of quartz; rutile markedly enhanced the toxicity of quartz, but did not significantly alter its transforming activity. Large thin plates of quartz were especially prepared from quartz crystals cut at different angles, representing different crystal periodicity: BALB/3T3/A31-1-1 cells, directly plated on these smooth quartz surfaces, grew well into continuous monolayers, were maintained for several weeks and then were cryopreserved. This method will be used to study effects of surface contact with quartz without cell internalization.